CHAPTERS FOR STUDENTS. 
653 
This number is the same for all gases; it should be recollected. 
9. When a body is heated it expands, that is, it occupies a larger space 
than before. It is, therefore, lighter than before; for any portion of it 
measured out contains less substance than before, and so weighs less. Ice, 
which is solidified water, expands in the process of its formation ; it is, 
therefore, lighter than the water in which it is formed, and, as said before, it 
floats. 
Hot water poured steadily upon cold water will remain uppermost for some 
time; and cold water introduced in a thin stream into a vessel of hot water 
will sink to the bottom. 
The fire-balloon consists of a balloon with a wide orifice below, in which is 
suspended a sponge wetted with spirit of wine. On setting fire to the spirit, 
the balloon, if not too heavy, will rise ; the air within becoming heated, is 
thereby expanded, hence it is lighter than the cold air surroundiug, and tends 
to rise through it. 
The smoke of a fire goes up the chimney by the same kind of action ; it is 
carried up by the warm, and consequently ascendiug, current of air. 
In measuring liquids, and especially gases (as in the determination of their 
specific gravities), it is, therefore, necessary to take into account differences of 
temperature, and to correct the results accordingly. 
10. Instruments by which we can measure the extent of expansion in certain 
selected bodies (usually liquids), upon increasing the heat in them, are called 
thermometers. The w r ord denotes “ heat-measurer but this is not strictly cor¬ 
rect, because we have no means of knowing for certain whether equal rise of 
temperature indicates truly equal increments of heat. 
The ordinary mercury thermometer consists of a very narrow tube terminated 
by a bulb, and sealed by fusion at the opposite end. The bulb contains mer¬ 
cury, which partly extends up the tube. It is obvious that if the quantity of 
mercury in the bulb is very much greater than that portion which extends into 
the tube, a very slight change in bulk, that is, in apparent quantity, will be 
shown by the movement up or down of the liquid in the stem. The thermo¬ 
meter is marked and graduated by ascertaining in the first place the points at 
which the mercury in the tube stands at two widely-separated temperatures, 
which are known to be invariable. These temperatures are those of melting ice 
and steam from boiling water.* 
11. The interval between these two points is divided into equal parts, called 
degrees ; the number of which is different, according to different systems. 
Those of Fahrenheit and Celsius (commonly called centigrade) are the only ones 
of importance here. The point at which the mercury stands in melting ice 
is made the zero, 0° upon the centigrade, and 32° upon the Fahrenheit thermo¬ 
meter. The interval between this and the higher point, ascertained by the dila¬ 
tation caused by exposure to steam, is made 100° centigrade or 180° Fahrenheit. 
With the latter, then, the higher point marks 212 degrees. 
100° centigrade mark the same change of temparature as 180° Fahrenheit; a 
degree centigrade is, therefore, greater than a degree Fahrenheit in the propor¬ 
tion of nine to five. 
To calculate any number of degrees upon the one scale into its equivalent 
upon the other, the following expressions should be recollected :— 
No. of centigrade degrees = (no. of F. deg. — 32) 
*/ 
Example : —To convert 50° F. into centigrade. 
C = | (50 - 32) = | X 18 = 10° 
* Under a pressure, as indicated by the barometer of about 30 inches of mercury. 
For the method of tilling, etc., see Attfield’s c Chemistry,’ p. 45«. 
